Method for recycling industrial waste streams containing zinc compounds

ABSTRACT

An industrial waste stream recycling method for recovery of high purity zinc oxide products and other chemical and metal values from industrial waste streams containing zinc compounds by leaching the waste stream with a solution of 30% or greater by weight ammonium chloride, resulting in a first product solution and undissolved materials; adding zinc metal to the first product solution, whereby zinc-displaceable metal ions contained in the first product solution are displaced by the zinc metal and precipitate out of the first product solution as metals, leaving a second product solution; and diluting the second product solution with water, resulting in the precipitation of zinc oxide.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a process for recovery of zinc productsincluding essentially pure metal oxides and metals. The presentinvention more specifically relates to an improved method for therecovery of essentially pure zinc oxide from EAF dust.

2. Description of Related Art

Zinc oxide typically is a fine white or grayish powder which has avariety of uses including as a rubber accelerator, as a pigment, as adietary supplement and in the semiconductor field. Zinc oxide is foundin commercial by-products including waste material streams such as flyash and flue dust. Methods for recovering zinc oxides are known in theart, including recovering zinc oxide from industrial waste materials.Such previous methods have included leaching with mineral acid, causticsoda, ammonium hydroxide, and ammonium carbonate solutions. However,these methods have low yields of zinc oxide and typically do not recoverpure zinc oxide, the recovered zinc oxide being contaminated with othermetal salts. Therefore, in order to obtain pure zinc oxide, subsequentreduction and washing processes were necessary.

U.S. Pat. No. 3,849,121 to Burrows discloses a method for the selectiverecovery of zinc oxide from industrial waste. The Burrows methodcomprises leaching a waste material with an ammonium chloride solutionat elevated temperatures, separating iron from solution, treating thesolution with zinc metal and cooling the solution to precipitate zincoxide. The Burrows patent discloses a method to take EAF dust which ismainly a mixture of iron and zinc oxides and, in a series of steps, toseparate out the iron oxides and waste metals. However, the materialobtained in the last step is a mixture of a small amount of zinc oxide,hydrated zinc phases which can include hydrates of zinc oxide and zinchydroxide, as well as other phases and a large amount of diamino zincdichloride Zn(NH₃)₂Cl₂ or other similar compounds containing zinc andchlorine ions. Currently, the Burrows method is not economically viablebecause of Environmental Protection Agency guidelines establishedsubsequent to the issuance of the Burrows patent. Additionally, theBurrows method is not a continuous method and, therefore, is noteconomical as a continuous process.

U.S. Pat. No. 4,071,357 to Peters discloses a method for recoveringmetal values which includes a steam distillation step and a calciningstep to precipitate zinc carbonate and to convert the zinc carbonate tozinc oxide, respectively. Peters further discloses the use of a solutioncontaining approximately equal amounts of ammonia and carbon dioxide toleach the flue dust at room temperature, resulting in the extraction ofonly about half of the zinc in the dust, almost 7% of the iron, lessthan 5% of the lead, and less than half of the cadmium.

Steam distillation is contrary to dilution. Steam distillationprecipitates zinc carbonate, other carbonates and iron impurities. Steamdistillation also disadvantageously results in an increase intemperature which drives off ammonia and carbon dioxide, resulting inthe precipitation of iron impurities and then zinc carbonate and otherdissolved metals. The purity of the zinc carbonate obtained depends onthe rate of steam distillation and the efficiency of solids separationas a function of time. Calcining converts the zinc carbonate to zincoxide, whereas washing and drying at temperatures between 100° C. and200° C. converts the zinc compounds to zinc oxide.

U.S. Pat. No. 5,464,596 to Myerson, commonly assigned with the presentapplication, discloses a method for the recovery of zinc oxide bytreating a waste stream with a 23% ammonium chloride at 90° C.,separating undissolved components from the solution, displacingundesired metal ions from the solution using zinc metal, cooling thesolution to precipitated out zinc compounds, washing the precipitate toremove various soluble zinc compounds, leaving zinc oxide of greaterthan 99%. The '596 patent teaches that ammonium chloride solutions mustbe at least 90° C. to sufficiently dissolve the zinc compounds. Heatingan aqueous solution to such a temperature requires the expenditure oflarge amounts of energy. It is further taught that while NH₄Clconcentrations below 23% do not dissolve the maximum amount of zincoxide from the waste material, concentrations greater than 23% result inan impure zinc oxide due to the tendency of the NH₄Cl to precipitate outof solution with the zinc compounds at such high concentrations.Furthermore, the cooling of the product solution results in theprecipitation of various zinc species, resulting in crystallization ofsome species. Because of this, using the cooling step disclosed inMyerson '596, one cannot use concentrations of ammonium chloridesolutions above about 23%, limiting the usefulness of the processdisclosed in Myerson '596. Further, contaminates of the zinc oxide mustbe removed by an additional washing step.

U.S. Pat. No. 5,759,503 to Myerson, et al., commonly assigned with thepresent application, discloses a method for the recovery of zinc oxideby dissolving zinc oxide in an intermediate, diluting the intermediateby a factor of 3 to 30 by adding 70-100° C. water, and filtering out theresultant zinc oxide crystals. The '503 patent, along with its family ofpatents, disclose using ammonium chloride solutions of 23% and teachthat using higher concentration ammonium chloride solutions will produceundesired results. This was the understanding at the time of inventionof the processes disclosed and claimed in the Myerson '503 patent andits family of patents. These undesired results include the precipitationof various zinc species, resulting in crystallization of some species.Because of this, using the cooling step disclosed in Myerson '596, onecannot use concentrations of ammonium chloride solutions above about23%, limiting the usefulness of the process disclosed in Myerson '503Although the processes disclosed and claimed in Myerson '503 patent arevaluable and perform admirably, it has now been discovered that byaltering the process steps, including the addition of a dilution step,ammonium chloride solutions of 30% or greater can be used to recycleindustrial waste streams.

Therefore, there exists a need for an alternative method that willrecover essentially pure zinc oxide from industrial waste materials thatis economical, quick, and efficient and, optionally also will allow therecovery of elemental lead, cadmium, and copper from industrial wastematerials, at the lower end of the temperature range and at higher leachsolution concentrations than previously thought possible.

BRIEF SUMMARY OF THE INVENTION

The present invention satisfies these needs in a method which recoversessentially pure zinc oxide from waste material containing zinc or zincoxide. Along with the essentially pure zinc oxide, zinc metal also canbe recovered, as well as values of other metallic elements originallycontained in the waste material, such as lead, silver, and cadmium. Thesolutions used in the process are recycled such that liquid wastes arekept to a minimum and, ideally, eliminated. The solids recovered fromthe process, namely, the zinc oxide, zinc metal, other chemical andmetal values, and other residues all can be used in other processes. Onesuch residue, an iron oxide cake, is of such a quality that it can beused directly as the feedstock for the typical steel production process.

Briefly, the present invention provides a method for recovery of highpurity zinc oxide products from industrial waste streams such aselectric arc furnace effluents (dusts, fumes, and vapors) containingzinc compounds comprising the steps of: (a) leaching the waste streamwith a solution of 30% or greater by weight ammonium chlorideat at leastabout 70° C., resulting in a first product solution and undissolvedmaterials; (b) adding zinc metal to the first product solution in acementation step, whereby zinc-displaceable metal ions contained in thefirst product solution are displaced by the zinc metal and precipitateout of the first product solution as metals, leaving a second productsolution; and (c) diluting the second product solution with water,resulting in the precipitation of zinc oxide and a third productsolution. The dilution step circumvents the crystallization of variousspecies, allowing the use of the higher concentration ammonium chloridesolution of approximately 30% or greater by weight. The variousundissolved precipitates produced during the process comprise both wasteproducts and chemical and metal values that can be recovered and sold,used in subsequent processes, or added to the feed to various industrialprocesses such as the iron and steel making processes.

The third product solution is concentrated after removing the zincoxide, resulting in a fourth product solution comprising greater than30% ammonium chloride. The fourth product solution then is combined withthe original ammonium chloride solution of Step (a) to leach theindustrial waste stream in a continuous process.

If the industrial waste material stream contains significant amounts ofiron, the waste stream preferably is heated in a reducing atmosphereprior to leaching, resulting in an iron-containing residue and acombustion product (dust, fumes, and/or vapors) waste stream comprisingoxides of zinc. The dust, fumes and/or vapors then are subjected to theprocess disclosed in this specification.

In yet another embodiment of the present invention, using a waste streamsuch as typical electric arc furnace dust (which typically containszinc, cadmium, copper, lead, and iron metals and compounds) theprecipitated metals will comprise zinc, lead, cadmium and coppercompounds. This alternate embodiment further comprises the additionalsteps of: (1) treating the precipitated metals with an aqueous solutionof either H₂SO₄ or NH₄SO₄, whereby zinc, cadmium, and copper compoundsgo into solution and lead compounds do not, resulting in a fifth productsolution comprising zinc, cadmium and copper compounds and a secondundissolved precipitate comprising lead compounds; (2) adding zinc metalto the fifth product solution, whereby cadmium and copper compounds aredisplaced by the zinc metal and precipitate out of the fifth productsolution as a third precipitate, leaving a sixth product solution; and(3) treating the sixth product solution with calcium chloride, resultingin the precipitation of CaSO₄ from the sixth product solution. Again,the undissolved precipitates produced during the process comprise bothwaste products and chemical and metal values that can be recovered andsold, used in subsequent processes, or added to the feed to variousindustrial processes such as the iron and steel making processes.

If it is desired to control sodium or potassium chloride concentrationsduring the process, additional steps of: (i) adjusting the pH of thesecond product solution to between about 5 and about 8 with NH₄OH priorto step(c); (ii) cooling at least a portion of the second productsolution to precipitate diamino zinc dichloride; (iii) evaporating thesecond product solution to precipitate NaCl and KCl; and (iv) combiningthe second product solution with the ammonium chloride solution to leachthe waste stream.

Therefore, it is an object of the present invention to provide a methodfor recovering zinc oxide from waste materials, such as fly ash or fluedust, which contain other metals, such as iron oxide, lead oxide,cadmium, copper and other materials.

Yet another object of the present invention is to provide a method forrecovering zinc oxide in which all leaching and washing solutions arerecycled for further use, and no leaching or washing solutions aredisposed of into the sewers or the environment.

Still another object of the present invention is to provide a method forrecovering zinc oxide which also results in the precipitation inelemental form of any lead, cadmium and copper metals contained in thestarting materials.

Another object of the present invention is to provide a method forrecovering zinc metal, zinc oxide and/or iron oxide which is economical,quick and efficient.

These objects and other objects, features and advantages of the presentinvention will become apparent to one skilled in the art when thefollowing Detailed Description of the Preferred Embodiments is read inconjunction with the attached figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow chart of the purification steps executed in thepreferred embodiment of the present invention.

FIG. 2 is a flow chart of the purification steps executed with theoptional reduction step, in accordance with an alternative embodiment ofthe present invention.

FIG. 3 is a flow chart of the purification steps executed to furtherisolate precipitated metals, in accordance with an alternativeembodiment of the presented invention.

FIG. 4 is a flow chart of the purification steps executed to controlsodium or potassium chloride levels, in accordance with an alternativeembodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, a preferred embodiment of the process is shown. Thepreferred waste materials such as fly ash or flue dust from electric arcfurnaces, as well as other waste materials streams, including othermetal producing and altering processes, contain economically valuableamounts of zinc compounds. However, these waste materials also containoxides of iron, lead, calcium, potassium, cadmium, and copper. Thepreferred waste is dust, solids, vapors and/or fumes from a steel makingprocess.

In step 100, a waste material is leached in an ammonium chloridesolution resulting in a first product solution and undissolvedmaterials. The ammonium chloride solution is greater than 30% ammoniumchloride in water at a temperature above about 70° C. Prior artconcentrations were typically not above 23% due to the tendency ofammonium chloride to precipitate out with zinc oxide. As explained inmore detail below, the present process has overcome such limitations.The majority of the waste materials, including any zinc and/or zincoxide, lead oxide, cadmium oxide, copper oxide, and other metal oxides,dissolves in the ammonium chloride solution forming a first productsolution. Iron oxide, also typically present in the preferred wastematerials, does not dissolve in the ammonium chloride solution, andremains as an undissolved material 102. Any other constituents in thewaste materials not soluble in ammonium chloride also remain asundissolved materials 102. The product solution is filtered and theundissolved materials 102 are removed 102.

If iron oxide is present in the waste materials, the undissolvedmaterials 102 separated from the first product solution typically isrich in iron oxide, and also typically has some impurities such as zincferrite. The undissolved materials 102 can be used as a feedstock forsteel mills so long as the quantity of impurities is not too great. Itis preferable to remove the impurities from the iron oxide prior tousing the iron oxide as a feedstock. Reducing the iron oxide in theundissolved materials 102 to direct-reduced iron (DRI) also is desiredas DRI can be used to replace part or all of the steel scrap charge.This is a first example of how even the waste from the present processis usable, resulting in an environmentally friendly process.

The iron oxide in the undissolved materials 102 can be reduced to DRI intwo manners. First, carbon, in the form of activated carbon, carbondust, carbon pellets or the like, can be introduced to the ammoniumchloride and waste material mixture during the leaching process. Thecarbon reduces the iron oxide resulting in DRI upon heating. Second, thecarbon can be introduced to the dried undissolved material cake using aribbon blender. The carbon will react with the iron oxide, reducing theiron oxide to DRI upon heating. Adding heat to this process assists inthe reduction.

The remaining first product solution contains zinc oxide and otherdissolved metal oxides; namely, oxides of lead, cadmium, and copper.While this first product solution is still hot, finely powdered zincmetal is added to the first product solution in a cementation step 104.Through an electrochemical reaction, any lead, cadmium, and copper metalin solution plates out onto the surfaces of the zinc metal particles.The addition of sufficient powdered zinc metal results in the removal ofmost of the lead, cadmium, and copper of the first product solution byprecipitation. After cementation, there is now a second product solutionand a precipitate 108.

The second product solution is rich in zinc compounds, while theprecipitate 108 is rich in oxides of lead, cadmium, and copper. Theprecipitate 108 is separated from the second product solution and, aswill be described in more detail below, can optionally be treatedfurther to recover the economically valuable metals. This is a secondexample of how the waste from the present process is usable, reducingthe amount of waste form the process.

Purified zinc may be recovered as either zinc oxide by diluting thesecond product solution in a dilution step 106 or as elemental zinc byelectrowinning (not shown). Recovery of zinc oxide by dilution ispreferred, as it has been discovered that this allows the use of higherconcentrations of ammonium chloride solutions of 30% or greater byweight. To recover zinc oxide in the dilution step 106, the secondproduct solution is diluted to a concentration between about 2% andabout 10% ammonium chloride. As the concentration of ammonium chloridein the second product solution drops, the solubility of zinc oxidedecreases, thereby resulting in the precipitation of zinc oxide from thesecond product solution. When diluting the second product solution, itis preferable to add the second product solution to the water, ratherthan adding water to the second product solution. The resultant zincoxide has significantly lower chloride content if the second productsolution is added to the water. After dilution, there is now a thirdproduct solution and precipitated zinc oxide 112.

The preferred method of dilution involves adding the second productsolution to the water, and not adding the water to the second productsolution. Preferably, the water should be at 60° C. or higher during theentire dilution step. The second product solution should be added to thewater in a gradual fashion. For example, in a batch system, one-third ofthe second product solution should be added to the total required amountof water and stirred to form a first intermediate solution. Theremainder of the second product solution, either in one large batch orin two or more smaller batches, should then be added to the firstintermediate solution to form the third product solution. This allowsthe ammonium concentration to be lowered to a relatively low level inthe first intermediate solution, forcing the dilution to proceed along adefined path. The zinc oxide precipitates out of the first intermediatesolution such that the concentration of ammonium chloride in the firstintermediate solution is even lower when the remainder of the ammoniumchloride solution is added to the first intermediate solution.

In another example, in a continuous system, a first fraction of thesecond product solution is added to water in a first vessel to form thefirst intermediate solution. The first intermediate solution should havea concentration of ZnO/NH₄Cl to water of between about 2% and 6%,preferably about 3.3%. Any zinc oxide precipitating out of the firstintermediate solution is removed from the first vessel. The first vesselis maintained at steady state at the desired concentration by removingany zinc oxide precipitate, adding additional water or second productsolution, and/or removing first intermediate solution. The removed firstintermediate solution is transferred to a second vessel where a secondfraction of the second product solution is added to the firstintermediate solution to form a second intermediate solution. The secondintermediate solution should have a concentration of ZnO/NH₄Cl to waterof between about 5% and 9%, preferably 6.6%. The second vessel ismaintained at steady state at the desired concentration by removing anyzinc oxide precipitate, adding additional second product solution,and/or removing second intermediate solution. The removed secondintermediate solution is transferred to a third vessel where a thirdfraction of the second product solution is added to the secondintermediate solution to form a third intermediate or final solution.The third intermediate or final solution should have a concentration ofZnO/NH₄Cl to water of between about 8% and 12%, preferably 10%. Thisalso allows the ammonium concentration to be lowered to a relatively lowlevel in the first intermediate solution, forcing the dilution toproceed along a defined path. The zinc oxide precipitates out of thefirst intermediate solution such that the concentration of ammoniumchloride in the first intermediate solution is even lower when theremainder of the ammonium chloride solution is added to the firstintermediate solution.

The use of thirds as the fractions of second solution added to eachvessel is preferred, but it should be understood that any fraction ofthe second product solution can be added to the total required amount ofwater at a time. The important criteria is that the amount of the secondproduct solution added to the total required amount of water be kept toa level allowing for the greatest amount of zinc oxide to precipitateout of the first intermediate solution. This also prevents the formationof tetra-amino-zinc-tetrachloride [Zn(NH₄)₄Cl₄], a low zinc content,fine needle crystal that is difficult to filter separate or wash. Theamount of water necessary, and the addition rates of second productsolution to the water, can be determined by one of ordinary skill in thebasic chemical arts without undue experimentation. Further, determiningthe total required amount of water is simple chemistry and well withinthe skill of the person of ordinary skill in the art of this invention.It simply is enough water to lower the concentration of ammoniumchloride in the second product solution, which can be determined bysimple chemical analysis, to a theoretical concentration of from about2% to about 10% ammonium chloride.

The third product solution is filtered to remove the precipitated zincoxide 112, which is placed in a drying oven at a temperature of over100° C. After a sufficient drying period, the resultant dry white powderis essentially pure zinc oxide.

If it is desired to recover elemental zinc, the second product solutionis subjected to electrolysis in an electrolytic cell containing an anodeand a cathode. The second product solution comprises zinc ions insolution as Zn²⁺, which will be electrodeposited on the cathode.Although it is preferable to have the cathode made from zinc metal,cathodes of other materials will allow the electrodeposition of zincmetal from the second product solution.

Any of the electrolysis cells discussed in the literature are suitable,as long as such cells are configured for the electrolysis of zinc ioncontaining solutions. The two electrodes of the electrolysis cells areconnected externally to a power supply capable of impressing a suitablevoltage across the electrodes. The zinc ions, being positive in nature,migrate toward the negative electrode, or cathode, where they combinewith electrons supplied by the external circuit to form neutral zincmetal atoms. When this happens, the zinc metal, in effect, electroplatesonto the cathode. By using a zinc cathode, the entire cathode can beremoved and used as necessary as a source of zinc. Alternatively, acathode on which electroplated zinc metal can be easily removed can beused.

If zinc oxide is precipitated from the second product solution bydilution, the diluted third product solution contains ammonium chlorideand other compounds. Rather than dispose of this solution, it ispreferable to produce a more concentrated (>30%) ammonium chloridesolution 110 that is recycled back to the leaching step 100. This can beaccomplished using evaporators or reverse osmosis membrane technology.This is a third example of how the waste from the present process isusable, reducing the amount of waste produced by the present process toa minimum.

From an economically competitive situation, the use of reverse osmosismembrane technology to concentrate the third product solution to obtaina concentrated ammonium chloride solution on one side of the membranewill save feed costs. Every so often, it will be necessary to back flushthe salts off the membrane to recover them for makeup use in the future.In essence, reverse osmosis membrane technology is using a pump to pumpa solution through a membrane, which is significantly lower in cost thanburning natural gas in an evaporator condenser to evaporate water from asolution. This technology is used to filter out sodium chloride and theminerals out of seawater to make distilled water.

Referring to now FIG. 2, if there is significant iron content in thewaste material, it is preferable to first heat the waste material in areducing atmosphere in a reduction step 200, prior to the leaching step100. The waste material is typically reduced at a temperature greaterthan 420° C., and preferably between 700° C. and 1300° C. The reducingatmosphere can be created by using hydrogen gas, simple carbon speciesgases, such a carbon dioxide, or by heating the material in an oxygencontaining gas in the presence of elemental carbon. The carbon, whichmay be in the form of powdered coke or coal, is mixed with the iron andzinc containing waste prior to the reduction step. Examples of reductionprocesses include rotary hearth and rotary kiln furnaces. In theseprocesses, the powdered mixture is formed into briquettes or pelletsbefore feeding to the furnace. The iron oxide is reduced to metalliciron and remains within the briquette or pellet. Zinc, lead and cadmiumare reduced and leave the briquette or pellet in vapor form which issubsequently oxidized on contact with oxygen. Alternatively, a mixturecontaining electric arc furnace dust may be added back to the electricarc furnace which operates under reducing conditions. Mixtures of wastematerials such as electric arc furnace dust, oily mill scale and otherflue dusts can be used to improve the value of the iron product. Thecarbon is preferably in the form of dust or pellets. Typically heatingtimes are from 4 minutes to 4 hours, depending on the method used.

The heating and reduction step 200 results in the reduction of the ironcompounds in the waste material into direct reduced iron (DRI), and thegeneration of volatile materials and combustion products. The DRI can befed directly into a steel mill as a feed source. The combustionproducts, in the form of exhaust dusts, vapors, and fumes, are recoveredby a filter process, such as a bag house or a wet scrubber. The zinc,lead, cadmium, and copper in the waster material are vaporized in theheating and reduction step 200 and comprise a majority of the exhaustdusts, vapors, and fumes. The heating and reduction step 200 is notnecessary if there is no significant iron content in the waste. Theheating and reduction step 200 is a fourth example of how waste from thepresent process (in this case iron oxides) is usable, further reducingthe amount of waste produced by the present process.

Once the waste material is reduced, the process as represented in FIG. 2continues just as the preferred embodiment represented in FIG. 1. Thecombustion product (the exhaust dusts, vapors, and fumes) are leached inleaching step 100, undissolved materials 102 are separated from theleachant, and the leachant (first product solution) is subjected tocementation step 104 to remove Pb, Cd, and Cu, for further processing108. The remaining solution (second product solution) is diluted withwater in a dilution step 106 to 2-10% NH₄Cl, thus prompting ZnO toprecipitate out of the solution. The now remaining solution (thirdproduct solution) is then concentrated back to >30% NH₄Cl in aconcentration step 110 and is recycled back into the process as aleaching solution.

Waste materials typically have varying amounts of lead, cadmium, andcopper metals contained therein. For various reasons, it is desirable toremove such metals from the waste materials, for example, to recycle thelead, cadmium, and copper, or to prevent their release into theenvironment. Referring to FIG. 3, the lead, cadmium, and coppercompounds removed from the first product solution during the cementationstep 104 as precipitates 108 can be recovered by first treating thecompounds with H₂SO₄ or (NH₄)₂SO₄ in a dissolution step 300. The zinc,cadmium, and copper compounds will go into solution while the leadcompounds will not. The lead compounds are filtered out 304, leaving asolution of zinc, cadmium and copper compounds 302.

The Zn, Cd, and Cu-containing solution 302 is then treated with zincpowder in a second cementation step 306 to result in theelectrodeposition of Zn and Cd on the zinc, which is then filtered out310, leaving a solution of zinc compounds 308. To this solution, calciumchloride is added. The calcium react with the sulfate present in thesolution from the H₂SO₄ or (NH₄)₂SO₄ to form CaSO₄, when precipitatesout. The CaSO₄ is filtered out 314 and the remaining solution can berecycled back to the leaching step to further recover zinc 312.

Referring to FIG. 4, sodium and potassium chlorides, introduced by theinitial waste materials stream, may accumulate in the various productsolutions, thus adversely affecting process performance. The chloridelevel may optionally be controlled by periodic removal of chloridesalts. This is preferably accomplished by adjusting the pH of the secondproduct solution in a pH adjustment step 400 prior to the dilution step106. This will neutralize the acidic effects of the chloride ions byforming reacting therewith to form ammonium chloride and water.

At least a portion of the pH-adjusted second product solution may beremoved through a purge stream prior to the dilution step 106 andallowed to cool in a cooling step 404. As the purged pH-adjusted secondproduct solution cools, a portion of the zinc compounds will precipitateout of the purged pH-adjusted solution as diamino zinc dichloride, whichis then filtered out in a filtration step 408. The use of a purge streamassists in the control of the concentration of sodium chloride and/orpotassium chloride in the second product solution. It is preferred tokeep the concentrations of sodium chloride and potassium chloride downand if these concentrations build up, a larger purge stream is used.

The remaining product solution (third product solution) is evaporated orotherwise concentrated in concentration step 110 to precipitate sodiumchloride and potassium chloride, which are filtered out in a secondfiltration step 410. The remaining concentrated solution can then bereintroduced into the process at the leaching step 100. In this way,chloride levels are maintained as to not adversely effect the formationof substantially pure zinc oxide.

The above description sets forth the best mode of the invention as knownto the inventor at this time, and is for illustrative purposes only, asit is obvious to one skilled in the art to make modifications to thisprocess without departing from the spirit and scope of the invention andits equivalents as set forth in the appended claims.

What is claimed is:
 1. A method for recycling industrial waste streamscontaining zinc compounds comprising the steps of: (a) leaching saidwaste stream with a solution of 30% or greater by weight ammoniumchloride maintained at a temperature above 70° C., resulting in a firstproduct solution and undissolved materials; (b) adding zinc metal tosaid first product solution, whereby zinc-displaceable metal ionscontained in said first product solution are displaced by said zincmetal and precipitate out of said first product solution as metals,leaving a second product solution; (c) adjusting the pH of said secondproduct solution to between about 5 and about 8 with NH₄OH; and (d)diluting said second product solution with water to between 2% and 10%ammonium chloride, resulting in the precipitation of zinc oxide and athird product solution.
 2. A method as claimed in claim 1, wherein saidsolution of ammonium chloride is maintained at a temperature above about70° C.
 3. A method as claimed in claim 2, wherein said second productsolution is diluted to between about 2% and about 10% ammonium chloridein step(c).
 4. A method as claimed in claim 3, wherein said thirdproduct solution is concentrated after removing said zinc oxide,resulting in a fourth product solution comprising greater than 30%ammonium chloride.
 5. A method as claimed in claim 4, wherein saidfourth product solution is combined with the ammonium chloride solutionof step (a) to leach said waste stream.
 6. A method as claimed in claim1, wherein said waste stream is heated in a reducing atmosphere prior toleaching, resulting in an iron-containing residue and a waste streamcomprising oxides of zinc.
 7. A method as claimed in claim 1, whereinsaid waste stream further comprises lead, cadmium, and copper compounds.8. A method as claimed in claim 7, wherein said precipitated metalscomprises zinc, lead, cadmium and copper.
 9. A method as claimed inclaim 8, further comprising the step of treating said precipitatedmetals with an aqueous solution of a compound selected from the groupconsisting of H₂SO₄ and (NH₄)₂SO₄, whereby zinc, cadmium, and coppercompounds go into solution and lead compounds do not, resulting in afifth product solution comprising zinc, cadmium and copper compounds anda second undissolved precipitate comprising lead compounds.
 10. A methodas claimed in claim 9, further comprising the step of adding zinc metalto said fifth product solution, whereby cadmium and copper compounds aredisplaced by said zinc metal and precipitate out of said fifth productsolution as a third precipitate, leaving a sixth product solution.
 11. Amethod as claimed in claim 10, further comprising the step of treatingsaid sixth product solution with calcium chloride, resulting in theprecipitation of CaSO₄ from said sixth product solution.
 12. A method asclaimed in claim 11, wherein said sixth product solution is combinedwith said solution of ammonium chloride to leach said waste stream instep (a).
 13. A method as claimed in claim 12, wherein said aqueoussolution is H₂SO₄.
 14. A method as claimed in claim 12, wherein saidaqueous solution is (NH₄)₂SO₄.
 15. A method as claimed in claim 1,further comprising the steps of: (i) cooling at least a portion of saidsecond product solution to precipitate diamino zinc dichloride; (ii)removing said diamino zinc dichloride from said second product solution;(iii) evaporating said second product solution to precipitate NaCl andKCl; (iv) removing said NaCl and KCl from said second product solution;and (v) combining said second product solution with said ammoniumchloride solution to leach said waste stream.
 16. A method as claimed inclaim 2, wherein said solution of ammonium chloride is maintained at atemperature of between about 70° C. and 100° C.
 17. A method as claimedin claim 1, further comprising the step of treating said precipitatedmetals with an aqueous solution of a compound selected from the groupconsisting of H₂SO₄ and (NH₄)₂SO₄, whereby zinc, cadmium, and coppercompounds go into solution and lead compounds do not, resulting in arecycle product solution comprising zinc, cadmium and copper compoundsand a second undissolved precipitate comprising lead compounds.
 18. Amethod as claimed in claim 17, wherein said industrial waste stream isheated in a reducing atmosphere prior to leaching, resulting in aniron-containing residue and a waste stream comprising oxides of zinc.